PROPERTIES OF THE RETINA. 339 



covery was made by Boll,* and the facts were afterward carefully 

 investigated by Kiihne.f The red pigment, known usually as 

 visual purple or rhodopsin, is found only in the external segments 

 of the rods; the cones do not contain it. In the fovea, therefore, 

 which has only cones, the pigment is entirely absent. The existence 

 of the visual purple may be demonstrated very easily. A frog is 

 kept for some time in the dark; it is then killed and an eye removed 

 and bisected equatorially. If the vitreous is removed from the pos- 

 terior half the retina may be detached by means of a pair of forceps. 

 When the operation is performed in red or yellow light, as in photo- 

 graphic work, the detached retina on examination by daylight i& 

 found to be a deep-red color; but after a short exposure it fades 

 rapidly, finally becoming colorless. If the frogs before operation 

 were exposed to strong daylight, the retina is found to be 

 colorless. A similar pigment is found in the eyes of man and 

 the other mammalia. It has been shown, moreover, that a photo- 

 graph may be made upon the surface of the retina by means of this 

 purple. If the head of a rabbit or frog that has been kept in the 

 dark for some time is exposed with proper precautions to the light 

 of a window, for instance, the part of the retina on which the image 

 of the window-lights falls will be bleached, while the parts upon 

 which the image of the window-bars falls and the surrounding areas 

 of the retina will retain their red color. A figure of such a retinal 

 photograph or optogram, as it is called, is represented in the accom- 

 panying illustration ( Fig. 142) . The visual purple has been extracted 

 from the rods by solu- 

 tions of bile salts, this 

 substance having the 

 power to discharge the 

 pigment from its com- 

 bination in the rods 

 in the same way as it 

 discharges hemoglobin t 2 



from its Combination Fig 142 ._Op t ogram in eye of rabbit: 1, The nor- 



in the red COrpUSCleS. mal appearance of the retina in the rabbit's eye: a, The 



entrance of the optic nerve; 6, 6, a colorless strip of 



The Solutions thus medullated nerve fibers; c, a strip of deeper color sepa- 

 i_, j rating the lighter upper from the more heavily pigmented 



Obtained are alSO lower portion. 2 shows the optogram of a window. 



bleached upon ex- 

 posure to light. We have in the visual purple, therefore, an un- 

 stable substance readily decomposed or altered by the mechanical 

 effect of the ether waves, and also, it may be said, by gross me- 

 chanical reactions, such as compression; and there can be little 

 doubt that the substance plays an important part in the functional 



* Boll, "Archiv f. Physiologic," 1877, 4. 



t Kiihne, "Untersuch. a. d. physiol. Inst. d. Univ. Heidelberg," vol. i, 

 1878. Also "The Photochemistry of the Retina," etc., translated by Foster, 

 London, 1878. 



